Cellulose occurs naturally and has a high hydrophilicity. Thus, a hydrogel composed of this cellulose has superior biodegradability and biocompatibility, and has been widely used in various fields of food, cosmetics, medical supplies and the like. In addition, a transparent hydrogel material for soft contact lenses has been recently developed using a cellulose hydrogel.
Conventionally, various acrylate derivatives having a hydroxyl group have been used as a raw material of hydrated soft contact lenses. Of such derivatives, poly(2-hydroxyethyl methacrylate) (PHEMA) has been most often used in view of the relatively superior transparency and machinability. What influences the performance of a soft contact lens most is oxygen permeability coefficient. In the case of a hydrated soft contact lens, an increased water content can raise the oxygen permeability coefficient. However, PHEMA hydrogel achieves only about 38% water content.
This became an incentive for the development of a cellulose hydrogel having a high water content by itself. In general, however, a cellulose hydrogel has insufficient transparency due to radical swelling and shrinkage during coagulation and regeneration of cellulose. In addition, a gel having a high water content tends to show insufficient mechanical strength.
Thus, there have been made various attempts to solve these problems and to provide, from cellulose, a highly hydrated hydrogel superior in transparency and having a high strength. For example, a hydrogel crosslinked using a crosslinking agent has been proposed (Japanese Patent Unexamined Publication Nos. 2-168958 and 5-237142). In both cases, however, the use of a crosslinking agent for chemical crosslinking poses a possible problem of effusion of the crosslinking agent and side reaction in the gel to change physical properties of the gel such as a decrease in the transparency.
As explained in the above, the cellulose hydrogel is associated with difficulty in handling of the gel and cannot be used with ease, which consequently resulted in a failure to provide a gel applicable to various uses. In the case of a cellulose hydrogel, moreover, one wherein the hydroxyl groups of cellulose constituting the gel are not chemically crosslinked, and which satisfies requirements in terms of transparency and mechanical strength has not been found.
The cellulosic materials have been typically used in water treatment and food processing, as well as recovery of valuable components such as medicines from microorganisms and culture media and the like, as a separation membrane such as an ultrafiltration membrane and a dialysis membrane. When a cellulose solution is cast into a film and the film is coagulated and regenerated without further treatment, the size of the pores formed in the membrane shows very wide distribution, which in turn leads to greater molecular cutoff to result in inefficient separation of the desired substance. The wider pore size distribution also brings about non-uniform structure of the membrane, causing low mechanical strength and a failure to separate under a high pressure. In addition, an attempt has been made to reduce the molecular cutoff and increase mechanical strength by drawing or crosslinking a cellulose membrane (see, for example, Japanese Patent Unexamined Publication No. 3-65224). Such method requires additional steps of drawing and crosslinking to ultimately complicate the production process.
It is therefore an object of the present invention to provide a transparent cellulose hydrogel having a densely packed and uniform structure, as well as superior transparency and mechanical strength, wherein the hydroxyl groups of cellulose constituting the gel are not chemically crosslinked, and a process thereof.